Ozone is utilized in a number of industrial processes, including drinking water and waste water treatment and disinfection, pulp bleaching, ozonolysis reactions in fine chemical production, and flue-gas denitrification.
Ozone is an unstable compound that decomposes to oxygen under ambient conditions and therefore it is not feasible to manufacture, transport or store in the manner used for many chemicals supplied through normal commerce. Rather, ozone must be produced at the point-of-use and at the time it is needed. Since ozone is a toxic material, it is generated only where and when it is required, in order to limit the possibility and potential impact of incidents.
Ozone is typically generated from oxygen utilizing a corona discharge. Oxygen is often used as the oxygen source for ozone generation and results in ozone concentrations of 10 to 15% by weight (balance oxygen) being produced. Air may also be used as the source of oxygen and produces ozone concentrations of 1.5 to 3% (balance air). For moderate to large ozone requirements, the total capital plus operating costs are typically less when oxygen is used as the oxygen source.
Ozone is often utilized at 10 wt % ozone with the balance being largely oxygen. It has been recognized that the re-use of the oxygen from the ozone/oxygen mixture generated by oxygen-based ozone generators can substantially improve the economics for ozone generation. Therefore, many schemes have been proposed for the separation of ozone from the ozone/oxygen output stream of the ozone generator, that allow oxygen to be recycled back to the ozone generator. In most of these schemes the separated ozone is displaced into the final ozone-utilizing process by means of an inert gas stream. For example, Balcar et al. Ozone Chemistry and Technology, pp 53 to 59, Advances in Chemistry; American Chemical Society, Washington, D.C. 1959 proposed the cryogenic liquefaction of the ozone, with re-evaporation of the condensed ozone into a carrier gas. Kiffer et al. (U.S. Pat. No. 2,872,397) and Cook et al., Ozone Chemistry and Technology, pp. 44 to 52, Advances in Chemistry; American Chemical Society, Washington, D.C. 1959 propose the use of a silica gel adsorbent to selectively adsorb ozone from the ozone/oxygen mixture after cooling of the stream, and with subsequent desorption of the ozone to the customer process by means of an inert gas, such as air, nitrogen, argon, etc., or by application of a vacuum.
In addition, the use of selective adsorbents for separation of the oxygen and ozone in the production stream have been proposed. This separation allows recycle of the unused oxygen to the ozone generator and an ozone product stream. For example, U.S. Pat. No. 4,786,489 targets reducing the costs for large scale ozone and teaches the use of a low temperature (−80 to −90° C.) ozone/oxygen separation unit that is purged with an impure nitrogen carrier gas stream containing oxygen and/or air to give the ozone product. U.S. Pat. No. 5,520,887 is directed at reducing the costs of ozone generation for pulp bleaching and teaches the use of a PSA oxygen generator to provide an enriched oxygen feed to an ozone generator, an oxygen ozone PSA to adsorb ozone and at the same time recycle oxygen to the ozone generator. The nitrogen rich waste gas from the O2 PSA is used to purge adsorbed ozone from the ozone oxygen PSA to the ozone consuming process. U.S. Pat. No. 6,030,598 describes the production of an ozone containing gas stream by subjecting oxygen to an electric discharge, adsorbing the ozone thus generated onto a solid adsorbent (such as zeolite) and recycling the oxygen containing stream leaving the adsorbent to the ozonising process. The oxygen adsorbed on the adsorbent is periodically desorbed by co-currently passing a purge gas over the adsorbent, with the desorbed oxygen recycled to the ozonizer. Ozone is desorbed from the adsorbent by a counter-current flow of purge gas and can then be used in the process needing ozone. A 3-bed (or multiple thereof) process and cycle is described that allows ozone and recycled oxygen to be produced continuously, but still requires each bed to experience a non-productive hold step within a full cycle. U.S. Pat. No. 6,197,091 describes the use of an ozone/oxygen membrane separation system in which ozone permeates through the membrane and is carried by a carrier gas, such as nitrogen, argon or CO2 into the ozone utilizing application, while the oxygen enriched stream is recycled to the ozone generator.
U.S. Pat. No. 6,916,359 describes a method of providing ozone at a pressure above atmospheric pressure using an ozone generator and an oxygen ozone PSA system. The unadsorbed oxygen from the PSA is recycled back to the ozone generator and the ozone product is carried to the ozone application by an inert gas stream at a pressure such that no further compression is needed. The carrier gas can be nitrogen, but is preferentially compressed air used also to feed a PSA oxygen generator that can be used as the oxygen source. U.S. Pat. No. 7,766,995 is targeted at reducing the cost of ozone utilized in the removal and capture of NOx from industrial flue-gas and other process streams. An oxygen ozone separation means allows recycle of oxygen back to the ozone generator and the use of clean dry air to carry the ozone into the industrial process. Optimum ozone production costs are achieved by recycling oxygen to the ozone generator, using the cheapest possible carrier gas to carry the ozone to the point of use, and reducing the power utilization in the ozone generator by operating the generator at lower ozone concentrations than normal (e.g., 6%).
Clearly, there have been many attempts to develop a process that reduces the cost of ozone generation from oxygen by recovering and recycling the un-utilized oxygen stream to the ozone generation, but these attempts have limited commercial application.
A more recent improvement belonging to the same applicant as the current invention was described in U.S. patent application Ser. No. 15/377,021 (filed 13 Dec. 2016). This application describes a method of continuous production of ozone and the recovery of oxygen in a purge cycle adsorption process. This method uses four adsorbent beds that operate is a sequential cycle including the following overlapping cycle steps.    Step a) An oxygen and ozone mixture from an ozone generator is fed to a first bed, where the ozone is adsorbed and the non-adsorbed oxygen passes through the first bed and is recycled to the ozone generator.    Step b) Rinse gas is provided to the first bed in a counter current direction to the flow direction of step a) to desorb ozone from the first bed and deliver the ozone to a customer process. The rinse gas is provided from a third bed (see Step d)).    Step c) A nitrogen-rich purge gas is fed to the first bed, again in the counter current direction to desorb any remaining ozone from the first bed and deliver the ozone to the customer process.    Step d) An oxygen and ozone mixture from the ozone generator is again fed to the first bed in the same flow direction as in step a) and ozone is adsorbed. In this step, the non-adsorbed oxygen acts as a rinse gas to displace any nitrogen-rich purge gas remaining in the first bed and this is then fed to the third bed that is now operating according to step b). This step prepares the first bed to repeat step a) and to restart the cycle for beds 1 and 3.    Step e) An oxygen and ozone mixture from the ozone generator is fed to a second bed, where the ozone is adsorbed and the non-adsorbed oxygen passes through the second bed and is recycled to the ozone generator.    Step f) Rinse gas Is provided to the second bed in a counter current direction to the flow direction of step e) to desorb ozone from the second bed and deliver the ozone to a customer process. The rinse gas is provided from a fourth bed (see Step h)).    Step g) A nitrogen-rich purge gas is fed to the second bed, again in the counter current direction to desorb any remaining ozone from the second bed and deliver the ozone to the customer process.    Step h) An oxygen and ozone mixture from the ozone generator is again fed to the second bed in the same flow direction as in step e) and ozone is adsorbed. In this step, the non-adsorbed oxygen acts as a rinse gas to displace any nitrogen-rich purge gas remaining in the second bed and this is then fed to the fourth bed that is now operating according to step f). This step prepares the second bed to repeat step e) and to restart the cycle for beds 2 and 4.
The cycles for the four adsorption beds operate such that steps e) to h) are offset in time with respect to steps a) to d). The beginning of steps a) and c) overlap with the end of steps e) and g) and the end of steps a) and c) overlap with the beginning of steps e) and g). A portion of the rinse gas may be vented to the atmosphere at the start of steps d) and h). Make-up oxygen is added and mixed with the recycled oxygen before being fed to the ozone generator. The mixture of recycled oxygen and make-up oxygen may be fed through a blower to increase pressure before being fed to the ozone generator. Any ozone that is present in the recycled oxygen is removed by passing the mixture of recycled oxygen and make-up oxygen through an inline ozone destruct unit prior to being fed to the blower. The cycle described can be considered a concentration swing adsorption cycle. The duration of steps a) and c) and e) and g) are equal ranging from 5 to 500 seconds; preferably ranging from 50 to 300 seconds, and more preferably ranging from 60 to 180 seconds. The duration of steps b) and d) and f) and h) are equal ranging from 5 to 90% of the length of step a), preferably ranging from 30 to 80% of step a).
This sequenced cycle of operation can be further described with reference to prior art FIGS. 1 and 2. FIG. 1 is a schematic of the four bed process used to recover oxygen from an ozone and oxygen gas mixture as described above and FIG. 2 is a schematic of the cycle steps for the four bed oxygen recovery process.
As shown in FIG. 1, make up oxygen is supplied through line 20 and is mixed with recycled oxygen from the adsorbent beds through line 40. The combined oxygen stream is fed through blower 9 to an ozone generator 7. Ozone is produced in the ozone generator at ozone concentrations of 1 to 30%, preferably 5 to 15%, more preferably 8 to 10% by volume (balance oxygen). The ozone and oxygen mixture is fed through line 24 to a manifold 30 for supply of the oxygen/ozone mixture to the bottom of the adsorbent beds. Ozone is adsorbed within the selected adsorbent bed or beds. Line 40 collects the oxygen that passes un-adsorbed out the tops of the selected beds and recycles this stream to be mixed with the make-up oxygen as noted above. Line 50 is a supply line for nitrogen rich purge gas to by supplied to the tops of the beds and that is used to desorb the ozone from such beds. Line 60 collects the ozone product from the bottom of the beds and delivers the ozone product to the customer process. A manifold 70 allows oxygen rich gas to pass from the top of one selected bed to the top of another selected bed in order to remove residual nitrogen from the other selected bed. Alternatively, contaminants or excess inert gases may be vented from manifold 70 through valve 6, preferably passing through an inline ozone destruct unit prior to being vented.
To explain the cyclical operation and valve sequences, an X may be used to represent an adsorbent bed, e.g. any of absorbent beds A, B, C or D shown in FIG. 1. Similarly, a valve number preceded by X is to be interpreted as representing that valve number for any one of the beds A, B, C or D. Each Bed X (X=A, B, C or D) has 2 valves controlling gas flows at the bottom (X4 and X5) and 3 valves at the top (X1, X2 and X3). For a given bed, X, only one valve at the top and one valve at the bottom is open at any one time. Valve X4 connects the bottom of bed X to the oxygen/ozone manifold 30, and valve X1 connects the top of bed X to the recycle oxygen line 40. Valve X3 connects the top of bed X to the nitrogen rich purge gas stream supply line 50 and valve X5 connects the bottom of bed X to the ozone product line 60 to be sent to the customer process. Valve X2 connects the top of bed X to the manifold 70 for transfer between two beds or alternatively for venting through valve 6.
As shown in FIG. 2, the operation of the ozone generation process can be explained as a sequence of five steps using the four bed oxygen recovery process described in this patent application. In step 1 of the process, labelled S1, feed gas from the ozone generator enters bed X through valve X-4. Ozone in the feed gas is selectively adsorbed on the adsorbent in bed X and excess oxygen to be recovered passes through the bed X and through valve X-1 and then to the recycle circuit. The recovered oxygen is mixed with makeup oxygen and the mixed oxygen is compressed by a blower to overcome the pressure loss in the system before being sent to the ozone generator as described above.
When bed X is saturated with ozone in S1, and just before the ozone starts to break through from the adsorbent bed X, step 2 (S2) of the cycle is initiated. A stream of nitrogen rich purge gas from another bed is fed to the top of bed X through valve X2. This purge gas comes from a different bed which has just switched to receiving oxygen/ozone feed gas according to step 4 (S4) and step 5 (S5) as will be further explained below. The purge gas causes desorption of the ozone from bed X with the desorbed ozone passing through valve X5 and sent as product ozone to the customer.
At the conclusion of S2, an external purge gas stream is introduced into the top of bed X through valve X3 in step 3 (S3). This external purge gas can be either dry air or any other dry nitrogen rich gas having a dew point less than −80° F. This purge gas desorbs ozone remaining in the adsorbent bed X and then again passes through valve X5 as product.
When the ozone has been sufficiently desorbed from adsorbent bed X, S3 concludes and feed gas from the ozone generator is again introduced through valve X4, which starts step 4 (S4) of the cycle. In S4, the un-adsorbed gas exiting the top of the bed X initially has more nitrogen content than that from S1, because of the nitrogen rich purge gas used in the purge step S3. This higher nitrogen content gas is utilized as the purge gas for a different bed passing through valve X2 as explained for S2. A portion of this higher nitrogen gas may be vented from the process through the vent valve 6 in order to prevent buildup of unwanted contaminants like argon, hydrocarbons or water in the recycle oxygen gas.
The venting of the contaminants during S4 lasts for a short duration and is the only difference between S4 and step 5 (S5). Therefore, during S5, the vent valve 6 is closed and the purge gas from the adsorbent bed X continues to flow as purge gas to a different bed. As steps S4 and S5 proceed the residual nitrogen in bed X is displaced by adsorbed ozone as well as un-adsorbed oxygen, so that by the end of step 5, bed X has returned to a condition where the restart of the cycle at S1 is appropriate.
The duration of step 4 depends on the nature and amounts of impurities that can be tolerated in the feed to the ozone generator. In some circumstances this step may be omitted or utilized only intermittently, e.g. once every predetermined number of cycles. The duration of step 4 plus step 5, or step 5 alone if step 4 is omitted, depends on the level of nitrogen required in the feed to the ozone generator.
The system and operation described in U.S. patent application Ser. No. 15/377,021 provides for recycle of up to 70% of the oxygen to the ozone generator and therefore significantly reduces costs of operation. Another positive feature of this system and operation is that if the optional step 4 is not utilized, then there are no waste streams diverted or vented to the atmosphere. Rather, there is 100% utilization of the make-up oxygen, CDA or nitrogen purge and the ozone generated. However, the use of the exiting gas during step 4 and step 5 to purge a different bed can lead to higher oxygen levels in the ozone product. This can be undesirable for several reasons, including that if oxygen levels are too high in the ozone product, there is a risk of fire or explosion, from oxygen passing through solvents used in the reaction or collecting in the head space of the reactor. The system and operation described above may lead to ozone product oxygen levels in the range of 25% to 40%. This is generally an acceptable level of oxygen for safe operation and for most typical industrial or water industry ozone applications. However, this level of oxygen in the product ozone stream may be unacceptably high for many fine chemical reactions and production applications. Further, this operation requires that the purge gas flow or CDA or nitrogen must change rapidly from full flow when more than one bed is operating in step 3 to 40% to 60% when only one bed is operating in step 3 and one bed is operating in step 2. These transitions from high flow to low flow and low flow to high flow occur simultaneously four times during the operation cycle described above. This requires a sophisticated control scheme for the purge gas flow, making the operation relatively complicated.
There remains a need in the art for improvements to ozone production with the recapture and recycle of oxygen, particularly for use with fine chemical reaction and production.